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Close encounters of a cometary kind

On 4 November 2010 the Deep Impact spacecraft swooped passed hyperactive Comet Hartley 2. Members of the science team spoke to Astronomy Now about the flyby, the comet's unusual characteristics, and the excitement in the control room as the first images were downloaded.

Read more about the flyby in Nick Howes' article Coma Chameleon, in the February 2011 issue of Astronomy Now (on sale 20 January).

Why did the team visit Hartley 2, and can you describe the key differences between it and Tempel 1?

After several attempts failed to locate and observe our original target, Comet Boethin, from the Earth, we chose Hartley 2 as our mission target. Hartley 2 was known to be much smaller and more active than any previously visited nucleus. The primary differences between the Tempel 1 [the spacecraft's first destination] and Hartley 2 are the subject of papers in preparation!

The science team will probably have years of work from this comet, but what were the first reactions when you all saw its unusual shape?

Elizabeth Warner: My first thought was "Wow, it looks like Itokawa!" To see the rough areas and smooth area in the middle, the jets on the ends was just amazing. My next thought was "What a fun comet!"

Lucy McFadden: A mission encounter is always a surreal experience. There was a sense of urgency to review the data so that we could insure its safe and complete return to Earth. We had to do this because we needed to erase the tape recorders and continue recording data acquired after the approach phase of the mission. So we were monitoring the comet for days before closest approach. I was working with students and young scientists monitoring returned data from the Medium Resolution Imager. We were looking for data drop outs that could be corrected by repeating the transmission of the data from the spacecraft. What we saw was the changing appearance of the comet before we could resolve the nucleus. Its gross shape changes as it rotates and different projections of the nucleus are seen against the dark sky. We didn't know what we were looking at and we were trying to determine how many jets were active on the surface. Was it one, two or three? And how was the comet rotating, like a lazy top, or a spinning ice skater?

The time between resolving the nucleus and the spacecraft's closest approach was faster than the time it takes to transmit the data to earth. Five of the closest images were sent back to earth almost immediately. We knew that the spacecraft had flown past the comet and we had only a few minutes to wait for the selected closest approach images. They appeared on the screen and we were happy to see a bright, crisp image of a dumbbell shaped comet. We all cheered, jumped up and down and hugged whoever was near by.....I think. To be honest, I don't remember what I did! I was relieved that the comet looked interesting and its beauty was astonishing. It made me feel good just looking at it.

Data continued to come down and display on the wall-length screens in our science operations center and the computer displays in front of us. All of a sudden I heard a group on the other side of the room cheering and calling out, "Snow, there's snow!" I looked over at the images from the High Resolution Imager, and sure enough there were all sorts of spots on the image. We almost missed the nucleus, but there was this "snow". That was unexpected and hence exciting.

Pete Schultz: We suspected that comet Hartley might be different (we only had four other comets for comparison), but we had no idea that it would be covered with chunks and acting like a champagne bottle (with CO2 spewing out one end). My immediate thought was just awe. This is why we go there: you have no idea until you explore.

We all saw Malcolm Hartley’s face on the TV broadcasts at the moment he first saw his comet up close, can you describe the emotions that the team all felt when they got to see it for the first time?

Elizabeth Warner: (pictured left) Jubilant! We knew we had an exciting comet already based on the radar observations from Arecibo and the observations of the jets from our own MRI and HRI during the Approach Phase. But to finally see it was extremely exciting. Fortunately, there was a slight lag between the images appearing on the projection screens in front of us and the data going through the pipeline, so we had a few minutes to enjoy and stare at the images in wonder before we had to get to work on analyzing them, prepping materials for the post-encounter press briefing, and in my case, getting them on the website for the rest of the world to see!

Pete Schultz: The team went crazy. Everyone had been watching as the spacecraft approached closer and closer. We saw a “peanut” with gas/dust coming out one end, but we couldn’t resolve it. Then the first five images were sent down. The science team was lined up in front of the screen (we didn’t see the first image immediately). So, there was casual chatter and positioning for the best view. Some were asked to get down on the floor (for those watching from their console behind). All this was just chatter until the moment that the first image flashed onto the projection screen. Wow. We turned and looked at each other in disbelief. It’s Borrelly! No, it’s like the asteroid Itokawa! Then we realized that it was a combination of all the comets we’ve seen…and more! Somehow, this was supposed to be simple.

Lori Feaga: I was used to the prime mission where we had some very low signal to noise spectra of Tempel 1 and really had to carefully analyze the spectra in order to determine the coma’s gaseous composition. As the spectra and images of Hartley 2 were received from the spacecraft, I remember thinking “Wow, this is great, we can see water, carbon dioxide, ice and jets in the raw data!”

Was the level of activity a surprise, given, that the comet barely reached its expected visual magnitude from Earth?

Earth-based observations at previous apparitions implied that comet Hartley 2 had to be very active for its size. While Hartley 2 is a factor 5 smaller than comet 9P/Tempel 1 (which means a factor of 25 in surface area and a factor of 125 in volume!), it produces about the same amount of gas per second as Tempel 1. However, most of the light produced by gas is not visible to the human eye. Visible light from comets comes mostly from sunlight reflected from dust around the comet, and this comet does not produce as much dust as other comets.

The surprise was only in what form the activity took, how localized it was and what the surface looked like, since these parameters were unconstrained by previous data and models spanned a wide range of scenarios. During approach to the comet it had already become clear, from Earth-based observations, that the comet was less active than at the last observed apparition (1997) by something like a factor of 2, which accounted for the comet barely reaching its predicted visual magnitude. Even chunks of ice in the coma had been envisioned previously, but these were a surprise primarily because they had NOT been seen in other comets that had been studied in situ.

You saw an unusual outburst of gas from the comet in Sept/Oct, can you explain more on this?

In mid-September, EPOXI observed a huge, long increase in the production of CN gas. What was very unusual about this event is that it was not accompanied by a rise in the dust spewed out by the comet. We still don't know much about this anomaly. All that we know is this event is quite different from anything we have ever seen from a comet. Our Hartley 2 observing campaign began on Sept 5 and ended on Nov 25, so we have a lot of data to analyze to help us understand how this comet works and to look for other more subtle events.

NASA and The University of Maryland have been doing terrific outreach with their Amateur Observers' Program (AOP), how has this helped the mission?

Elizabeth Warner: The AOP is a bit of a pet project for me. I really enjoyed the interactions with the amateurs during the prime (Deep Impact) mission when we first put the AOP together. And while Tempel 1 was very faint, we still got some very good images. While the AOP concentrated on just collecting observations whether photographic, digital, sketch or visual reports, its sister program the Small Telescope Science Program (STSP) actively sought scientific observations from the amateurs. Data had to be processed a certain way, taken with certain filters. For EPOXI, we *technically* didn't have funding to do either, but I kind of snuck in doing the AOP between my duties as EPOXI mission webmaster. We've already collected more images than were submitted of Tempel 1. And while most of the observations are *pretty*, several amateurs have made their data also available to the team as well as other amateurs for analysis.

EPOXI not only did the two flybys of Comet Tempel 1 and Hartley 2, but also has been working on exoplanet detections, how is this work related to some of the more detailed ground based research in this field?

EPOXI spent most of the spring and summer of 2008 staring for a few weeks at a time at stars that are orbited by planets that transit the parent star. We already knew of those planets from ground-based observations. However, because the spacecraft is in orbit around the Sun instead of being on Earth or in Earth orbit, it is very thermally stable and steady and completely free from atmospheric perturbations, so it can make extremely precise measurements of these stars' brightness. These measurements determine the planets' size and details of the orbit with great accuracy. Some results already have been published, others are in press or in preparation. Because the spacecraft stared at each system for weeks at a time, the observations were sensitive to other planets that might transit, and in one case EPOXI was sensitive to planets comparable in size to our Earth. Although no Earth-like planets were found, the search prompted EPOXI investigators to develop new data analysis techniques that are being used to sensitively search for "super-Earths" in exoplanet data from the Spitzer Space telescope. The EPOXI mission also laid some groundwork for eventual direct imaging of terrestrial exoplanets by making several observations of the Earth and also of Mars in 2008 and 2009. These measurements will help to understand the possibilities and the limitations for deducing the properties of terrestrial exoplanets from high-contrast imaging. One of the most interesting conclusions from the EPOXI Earth observations was the demonstration that the presence of oceans and continents could be inferred from the multi-wavelength light curve of the rotating Earth. Some day we may find another Earth-like world, and be able to determine whether it has oceans and land masses like our home planet, even though it may be too distant to map them directly.

The spacecraft had two onboard cameras, yet up to a week after closest approach, only images from the medium resolution (MRI) one had been published. Why was the MRI thus able to get better focus on the comet than the HRI?

The mirror on the HRI is not positioned properly and hence is out of focus. This is a problem that we recognized shortly after the spacecraft was launched. There is no focusing mechanism on the telescope, so we effectively focus with deconvolution techniques once the data are transmitted to Earth. Unlike the HRI, the MRI is in focus. Without HRI deconvolution, the effective resolutions of the two cameras are about the same. Once the HRI images are carefully deconvolved, the HRI images are mostly restored and as shown in the released image in the second press conference, the HRI will help us understand the surface with its higher resolution.

Although it does take some time to deconvolve the HRI images and that’s partly why we didn’t release them right away, we also saw/recognized the amazing science with the snowpuffs and wanted to make sure we released the images (which we did) in the second press briefing… In fact, that’s kinda what drove us to have the second press briefing just two weeks after encounter. We didn’t do that with Deep Impact!
As revealed by Emily Lakdawalla's (blog article), the HRI “missed” seeing most of the comet. Did you notice how the comet is not centered in the MRI images? That means the HRI missed seeing it in several shots. But this was a fortunate mistake – although the comet is just barely in or on the edge of the field of view in the HRI, that allowed us to see/concentrate on the snowpuffs.

What are the plans for the future with regard to the spacecraft, is there enough fuel, though it’s limited, for any course corrections, or will “Isaac Newton be in the driving seat”?

Future observations, if approved by NASA, will likely be projects that can make observations from a distance and will take advantage of the long cruise times, much like the EPOCh half of this mission did in the early part of the mission.

And other projects, after the huge success of this mission and stardust, it seems like JPL have got comet encounters down to a fine art. What missions do you think will carry us forward into the next phase of comet research, and what (with no budget limit) would be the dream mission?

Comet scientists are looking forward to the Rosetta mission that will orbit and land on comet 67P/Churyumov-Gerasimenko in 2014 and watch its evolution as it approaches the Sun. Better yet, it would be scientifically valuable to return to the same comet after each close pass to the sun so that we can watch long-term changes on the surface. We will do this with Stardust/NExT to Tempel 1 in Feb. 2011, but have no current plans to do this on a continuing basis with a specific comet. During NASA’s most recent call for Discovery class mission proposals, several cometary missions were proposed. Planetary scientists who study small bodies have defined a need for a cryogenic sample return mission to a comet to best study the pristine materials left over from the formation of the Solar System.

Read more about the flyby in Nick Howes' article Coma Chameleon, in the February 2011 issue of Astronomy Now.

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